"I have no mouth yet I must scream" is by Harlan Ellison. And yes, the all-encompassing godlike computer is called AM -- as in "I Am". Whatever abbreviation "AM" started out, it became irrelevant when the computer decided it was God. Or close enough, anyway.

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Fiction has to be plausible. Reality is under no such constraint.

Not "yet", "and".

The aliens in Robert Silverberg's short story, "Passengers"; they temporarily--and totally at random--take over a person's body, forcing them into a fugue state in which they can do all sorts of atrocities that the victim doesn't even remember.

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"He fell for her like his heart was a mob informant and she was the East River."

--Anonymous

Haunted houses on a mega scale...

I don't know about "scariest", but the Knnn from Cherryh's Chanur saga are pretty darn creepy.

How about the not-ill-intentioned but super-fast-reproducing and compulsive "repairing" watchmakers of Mote in God's Eye fame?

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Todd (Bowie, MD, US, North America, Earth, Sol System, Vega region, Local Bubble, Orion arm, Milky Way Galaxy, Local Group, Virgo A Cluster, Virgo supercluster, the universe in which spock is clean shaven)

Quidquid latine dictum sit, altum sonatur.

personal page: http://blog.astrosketches.info

I have been wracking my brain (ow) but cannot think of the title or author of a certain short story. (Maybe someone here will recognize it?) The scariest alien comes from that story, and of course it's a human.

A man is sent to another planet on a diplomatic mission to negotiate an agreement with the planet's ruling committee. He is told that to speak with the commitee he must submit to various trials. These include ritual amputations of - first - fingers and toes, leading to his giving up his arms, legs, and genitals. Finally, he is blinded.

Then he is considered worthy enough not only to speak with the committee but to join it.

Shortly after, a servant informs him that his wife has arrived and wants to speak with him. He tells the servant to cut off one of her fingers.

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Never attribute to malice what can be adequately explained by ignorance or stupidity.
Isaac Asimov

I found the moties in general to be very scary. Just the fact that they were individually so dern likable, & collectively the biggest threat our species had ever encountered. Hmmmmm. Kinda like us?

More like Texans.

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The Devil offered me power. I told him I preferred aperture.

...and one Wyomingian.

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